1. Field of the Invention
The present invention relates to an RFID, a wireless tag, or a wireless network node that has an ID number or is equipped with a sensor.
2. Description of the Related Art
A small wireless network is generally called a wireless tag or an RFID. The small wireless network is divided into a passive type that does not have a battery, and an active type that requires a battery. The passive type may be regarded as a bar code system. The passive type RFID is mainly used when a master unit and a slave unit are disposed close to each other by several centimeters.
The passive type does not have a power supply and is supplied with energy from radio wave to be transmitted. Meanwhile, a so-called small wireless network node that is used in a sensor network is generally called the active type. The small wireless network node requires a battery but includes a transmitting/receiving circuit, thereby transmitting data in a wireless manner through a sensor. The wireless network node can perform data exchange with a network through the wireless network node itself. In addition, the active type has a communication distance much longer than the passive type, for example, tens of meters.
The passive type does not have a battery but needs a large antenna to increase a communication distance. In this case, strong radio wave may adversely affect other wireless apparatuses. The active type is appropriately used when a predetermined communication distance is needed in the wireless tag, but there is a problem in that a battery is needed. Accordingly, when battery consumption is low, and a battery does not need to be replaced for one year or more, the active type is often used, instead of the passive type.
In the related art, there are many disclosures on collision in the passive type (for example, Patent Document 2), but no disclosures on the active type. In addition, a power saving method is disclosed in Patent Document 1. According to this method, the same receiving circuit is used. Specifically, a power saving mode is changed to a normal mode at the first reception, and communication is performed at the second reception. In contrast, the present invention uses a communication procedure that can achieve anti-collision and power saving.
Patent Document 2: Japanese Patent Application Publication No. 2003-115033
Patent Document 2: Japanese Patent Application National Publication No. 2002-519876
The passive type wireless tag does not require a battery but has a short communication distance. In addition, a large transmitting antenna is needed to increase the communication distance, which causes an increase in transmission power. The increase in the transmission power adversely affects neighboring wireless apparatuses.
The active type wireless tag has a communication distance longer than the passive type, but it requires a battery. However, even though the battery is required when power consumption is reduced, the active type can be used according to the passive type. In order to reduce battery consumption, a low-rate CPU needs to be used, and thus it is necessary to perform power saving control in internal circuits.
To this end, it is necessary to allow even a low-rate CPU to perform wireless tag communication using the most simplified procedure. In addition, when a plurality of wireless tags exist in a communication area, the tags need to be individually identified. For this reason, there is a need for a communication procedure that can achieve power saving or a power saving method.